The present invention relates generally to image forming equipment and is particularly directed to a printing apparatus of the type which includes a single-pass intermediate transfer member, with multiple print engines. The invention is specifically disclosed as a printer that controls the print media-to-image registration where the unfixed image resides on a single-pass intermediate transfer member, and additionally reduces waste toner by detecting a sheet of print media in the input media source before imaging is started for that particular sheet.
The field of electrophotographic (EP) printers, particularly those intended for the office environment, is actively migrating from monochromatic (black) printers to color printers. This migration is being driven in part by the advent of systems that can deliver color prints at the same speed as black-only prints. Most conventional color EP printing machines can only produce color output at some percentage of the total output achievable for black (monochrome) output. This is dictated by conventional processes whereby a revolving member transfers a single color plane at a time to a receiving member.
Hence, for a typical color EP machine, the output printing rate for color print-outs is roughly cut by a factor of four. This type of architecture yields extremely lengthy time between prints, or extremely long interpage gaps between prints. Not only does the increase in interpage gap produce undesirable and excessive cycling of EP components, the increase in process times required to produce color copy at a rate deemed acceptable by the market with respect to black copy can significantly impact the machine size and cost.
Much recent activity within the prior art has concentrated on developing printer architectures that can layer the four color planes (cyan, magenta, yellow, and black) required for color imaging in a single pass, such that no penalty is paid for transferring individual color planes one at a time. Such single-pass printers layer the four colors either to an intermediate transfer member (ITM), or directly to the print media in a manner similar to more traditional EP printing devices.
One such architecture calls for print media to pass directly underneath four independent photoconductive drums arranged in tandem, whereby at each station a different color plane is laid on top of the foregoing plane. In this architecture, the leading edge of the print media is typically sensed just prior to the first imaging and toner development station at a sufficiently large distance that careful design and control of the timing and print media velocity, from the sense point to the transfer point, will lead to proper registration of the image with respect to the print media.
Another conventional printer design uses xe2x80x9cstaging,xe2x80x9d or xe2x80x9cgating,xe2x80x9d as a technique for aligning the print media with the image. This technique is not limited to color EP processes; the IBM 4028 monochrome laser printer utilizes a similar technique for reducing the time-to-first-print, and for marginally improving throughput. Basically, a sheet is conveyed and stopped at a known location at a known time, and then released at just the appropriate moment and accelerated to the correct velocity such that alignment with an image can occur accurately at a transfer station.
In an EP color printer having a single-pass or pipeline architecture, including an ITM belt or drum, the four color planes are layered first onto the intermediate member before the aggregate image is transferred to the print media at a secondary transfer point. In this arrangement, conveyance of the print media must be coordinated with image generation in a much more controlled fashion, especially for printing machines developed for the office environment where machine size is an important factor, which dictates that paper path lengths be minimized. This extra attention to print media registration and image placement is warranted due to the fact that the ITM circumferential travel distance can easily be two or three times the length of the typical input print media pathway, between the media picking point and the transfer nip (transfer point).
In order to image with a minimal yet known interpage gap between prints, the imaging process may have to be initiated well before the targeted sheet for that image is launched from the sheet feeder tray. Moreover, further complications are added to the printing system if optional sheet feeding units are added underneath the printer, such that the input print media pathway becomes longer than the image path. In this situation, sheet launching may have to take place prior to imaging depending on the geometry of the feeder. Whatever control mechanism that is put in place must be able to accommodate either pick-to-image scenario. These constraints pose a number of problems from a system control perspective.
One problem can be anticipated by considering that most sheet feeders introduce some amount of error in the location of the sheet at the time of the pick, due to print media slippage or velocity variation. Furthermore, if the sheet feeder incorporates a pick mechanism that tracks with the top of a varying print media stack height, the total distance the paper has to travel from its rest position to the transfer point (transfer nip) varies, sometimes unpredictably, as a function of the amount of print media within the tray. Other errors that introduce print media positional uncertainty result from velocity variation, print media slippage or deviation away from the nominal print media trajectory within the designed channel (e.g., an input print media pathway) where the print media is conveyed between the sheet feeder tray and the transfer nip. If all these error sources or uncertainties are present within a printing machine that embodies the ITM architecture, it can easily be understood that some method of controlling or eliminating these variances must be developed in order to properly register the top writing line of the print media with the image, assuming the location of the leading edge of the image is known within a good degree of certainty.
Another problem that arises as a result of developing an image onto the ITM before the print media is launched from the sheet feeder tray is in regard to waste management. Certainly it is to be expected that, with the ITM architecture, toner will need to be cleaned off of the transfer medium surface in the event of a feeding failure or jam, since the print media will not be present to receive the image. Failure to clean the ITM surface would otherwise result in contamination of future print jobs or other mechanical components within the machine. Fortunately, the art has progressed to the state that such occurrences can be minimized to the point of rarity.
However, with an ITM single-pass architecture, additional toner waste can be generated if a media-out condition is not detected prior to imaging, and toner accumulation being initiated on the ITM. If the media-out condition is not considered as a design criterion, the potential for this occurrence is certainly good given the difference in length of the ITM surface travel with respect to the input print media pathway length. For instance, it is conceivable that development of the second image of a multi-sheet print job could be well under way on the ITM by the time the first sheet in the sheet feeder tray has progressed far enough to allow detection of the availability of a second sheet in the tray (to receive the xe2x80x9csecondxe2x80x9d image already being developed on the ITM). If and when this condition occurs, waste toner would have to be sent to a volumetrically limited waste container.
Other waste management means potentially might lessen this impact to the ITM waste container. For instance, a sheet from an alternate auxiliary source might be launched on command from the controller with the express purpose of receiving the waste image, or the transfer bias voltages on the cartridge photoconductive drums might be reversed in order to back transfer the waste image and send it to the cartridge cleaner reservoir.
Less obvious is the fact that the registration problem and the waste management problem are coupled. If the design constraint to positively detect the presence of print media in the designated sheet feeder tray prior to imaging and toner accumulation is incorporated in the printer (due to a need to minimize waste), then the nature of the positional error correcting implementation must also operate under that same constraint.
Accordingly, it is a primary advantage of the present invention to provide an image forming apparatus that controls positional error in its output printing device, by applying an image onto an intermediate transfer member development using one or more print engines, while controlling the intermediate transfer member at a substantially predetermined velocity, which may be a constant velocity; and by controlling a variable velocity of a sheet of print media as that sheet travels through an input print media pathway so as to cause a leading edge of the sheet to arrive at the transfer nip at substantially the same moment as a leading edge of the image traveling on the intermediate transfer member, thereby substantially eliminating registration error.
It is another advantage of the present invention to provide an image forming apparatus that reduces waste toner by, prior to imaging at any of a plurality of print engines and prior to initiating a picking operation at an input print media source, determining whether or not at least one sheet of print media currently exists within the input print media source, and if not, preventing imaging.
It is a further advantage of the present invention to provide an image forming apparatus that corrects or minimizes the positional errors, as discussed above, without violating the need to prevent any waste toner from being generated as a result of a common media-out condition.
Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, a method for controlling positional error in an image forming system is provided, in which the method comprises: providing an image forming apparatus having a memory circuit for storage of data, and a processing circuit for controlling at least one physical device; providing at least one print engine, an intermediate transfer member, and an input print media pathway, wherein the intermediate transfer member and input print media pathway physically pass through a transfer nip; applying an image onto the intermediate transfer member by way of the one or more print engines, while controlling the intermediate transfer member at a substantially predetermined (perhaps constant) velocity; and controlling a variable velocity of a sheet of print media as that sheet travels through the input print media pathway, without halting movement of the sheet, so as to cause a leading edge of the sheet to arrive at the transfer nip at substantially the same moment as a leading edge of the image traveling on the intermediate transfer member, thereby substantially eliminating registration error.
In accordance with another aspect of the present invention, a method for reducing waste toner within an image forming system is provided, in which the method comprises: providing an image forming apparatus having a memory circuit for storage of data, and a processing circuit for controlling at least one physical device; providing a plurality of print engines and a single-pass intermediate transfer member; providing an input print media source, and an input print media pathway; and prior to imaging at any of the plurality of print engines and prior to initiating a picking operation at the input print media source, determining whether or not at least one sheet of print media currently exists within the input print media source, and if not, preventing imaging to reduce toner wastage.
In accordance with a further aspect of the present invention, an image forming apparatus is provided, comprising: a memory circuit for storage of data, and a processing circuit for controlling at least one physical device, at least one print engine, an intermediate transfer member, and an input print media pathway, wherein the intermediate transfer member and the input print media pathway physically pass through a transfer nip; wherein the processing circuit is configured to use the at least one print engine to apply an image onto the intermediate transfer member, while controlling the intermediate transfer member at a substantially predetermined (perhaps constant) velocity; and to control a variable velocity of a sheet of print media as that sheet travels through the input print media pathway, without halting movement of the sheet, so as to cause a leading edge of the sheet to arrive at the transfer nip at substantially the same moment as a leading edge of the image traveling on the intermediate transfer member, thereby substantially eliminating registration error.
Still other advantages of the present invention will become apparent to those skilled in this art from the following description and drawings wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.